The primary differences between .223 Remington and 5.56 x 45 mm (NATO) are that .223 Remington is loaded to lower pressures and velocities compared to 5.56 NATO and the 5.56 NATO chamber has a longer leade. .223 Remington ammunition can be safely fired in a 5.56 NATO chambered gun, but the reverse can be an unsafe combination. The additional pressure created by 5.56 NATO ammo will frequently cause over-pressure problems such as flowing brass, difficult extraction, or popped/punctured primers, but in extreme cases, could damage or destroy the firearm.

Additionally, SAAMI also says that we should not use 5.56 military ammo in rifles chambered for .223 Remington.